Hydrogen isotopes in leaf and tree-ring organic matter as potential
indicators of drought-induced tree mortality
Abstract
The oxygen and hydrogen isotopic composition (δ 18O, δ
2H) of plant tissues integrates hydrological,
physiological, and metabolic functions differently, which may allow
disentangling reasons of tree mortality. To test this, we performed a
greenhouse study and determined predisposing fertilization and lethal
drought effects on δ 18O and δ 2H
values of plant water and organic matter (OM) in leaf and woody tissues
of living and dead saplings of five European tree species. Additionally,
we measured physiological and metabolic traits. Compared to controls,
drought reduced leaf gas-exchange, predawn water potential, and stem
starch concentrations and increased δ 18O and δ
2H values of leaf and twig water in all tested
species. These drought-induced changes generally caused an
2H-enrichment in leaf and tree-ring OM, but a low and
heterogenous δ 18O response. δ 2H
values of tree-ring OM were correlated with those of leaf and twig water
across treatments and species. In contrast, the predisposing
fertilization had generally no significant effect on any isotopic,
physiological, and metabolic traits. We propose that the
2H-enrichment in the dying trees is related to (i) the
plant water isotopic composition, (ii) metabolic processes shaping leaf
non-structural carbohydrates , (iii) the use of carbon reserves
for growth, and (iv) species-specific physiological adjustments. This
stress imprint on δ 2H but not on δ
18O suggests that the further could be used as a proxy
to understand mechanisms of drought-induced tree mortality.